Oscillators including a resonator integrated in a negative resistance element have been widely used as current injection light sources which generate electromagnetic waves (hereinafter referred to as “terahertz waves”) in a frequency domain from a millimeter waveband to a terahertz waveband (from 30 GHz inclusive to 30 THz inclusive). PTL 1 discloses an oscillator for terahertz waves which includes a double barrier resonant tunneling diode (hereinafter simply referred to as an “RTD” where appropriate) serving as a negative resistance element and a micro-strip resonator which are integrated in the same substrate.
Oscillators using a negative resistance element usually generate parasitic oscillation caused by a bias circuit which includes a power source and wiring which are used to control a bias voltage of the negative resistance element. The parasitic oscillation generated in a lower frequency band other than a desired frequency deteriorates oscillation output in the desired frequency.
To suppress the parasitic oscillation, assuming that a wavelength of a terahertz wave oscillated by the oscillator is denoted by λTHz and an oscillation frequency is denoted by fTHz, impedance on a bias circuit side is reduced in a frequency domain equal to or larger than DC and smaller than fTHz. To address this problem, a method for arranging a circuit including a resistance and a capacitance in a position within λTHz/4 on a power source side viewed from the RTD has been proposed.
As a device which realizes this method, NPL 1 discloses an oscillator employing a slot resonator. In NPL 1, as illustrated in FIG. 11, a rectifying diode 15 is disposed in a position within λTHz/4 on a power source 16 side viewed from S-RTD 11. Note that a resistance 17 is obtained by adding an internal resistance of the power source 16 and a resistance of a connecting wire to each other.
The method employed in NPL 1 is employed only for slot resonators, and therefore, it is difficult to employ the method in oscillators employing a microstrip resonator, such as a patch antenna disclosed in PTL 1. This is because, if the resonator is a patch antenna, for example, a region within λTHz/4 on a bias circuit side viewed from a negative resistance element includes the patch antenna or peripherals of the patch antenna which are arranged close to the patch antenna. Therefore, in microstrip resonators, it is difficult to arrange a circuit such that the circuit does not interfere with a resonator.
Furthermore, since a line structure for controlling a bias voltage of the negative resistance element is arranged close to the resonator, parasitic oscillation of comparatively high frequencies caused by the structure is required to be reduced.